Integrated circuits and other semiconductor devices often include long chains of vias, which are referred to as “via chains.” A via may, for example, be a vertical metal interconnect between two planar metal layers. Each via in a via chain typically connects one conductive region to another conductive region. A via chain may include any number of vias, such as one thousand vias or more. After a manufacturer produces the semiconductor device, the via chain is typically tested to ensure that the vias have been manufactured properly.
One problem in the manufacture of the via chains is the presence of “open” vias. An “open” via typically fails to satisfactorily connect two conductive regions. Because of this, the via chain cannot conduct adequately from one end of the chain to the other end of the chain. The absence of a proper connection between two conductive regions in the chain prevents the via chain from operating properly.
Another problem in the manufacture of the via chains is the presence of “partially open” vias. A “partially open” via typically connects two conductive regions, but the resistance of the via is higher than expected. As a result, the via chain may be able to conduct from one end of the chain to the other end of the chain, but the higher resistance of the via may still interfere with the operation of the via chain.
Testing a via chain for open vias is often a simple task. To detect if a via chain includes any open vias, manufacturers typically measure the resistance of the entire via chain. If the resistance of the entire via chain is extremely or infinitely high, the via chain probably includes one or more open vias. Because current flow through the via chain is significantly restricted, the measured resistance appears extremely large.
Testing a via chain for partially open or resistive vias is typically a more difficult task. To test for partially open vias, manufacturers again typically measure the resistance of the entire via chain. Because each via typically has an expected resistance, the manufacturers can usually determine an expected resistance of the entire via chain. For example, vias may have an average resistance of between eight and twelve ohms, so a via chain having one thousand vias has an expected resistance of between eight thousand and twelve thousand ohms. By measuring the resistance of the entire via chain, the manufactures may be able to identify when a via chain contains a very high-resistance via, such as when a via has a resistance of one thousand ohms.
A problem with this approach is that the manufacturers may still fail to identify via chains containing vias that have much larger than expected resistances. For example, in the via chain having one thousand vias and an expected total resistance of between eight thousand and twelve thousand ohms, one via could have a resistance of one thousand ohms, while the other vias have expected or lower than expected resistances. Even though this one via is a partially open via, the resistance of the entire via chain could still fall within the expected resistance range. For instance, if each of the other vias has a resistance of nine ohms, the resistance of the entire via chain is approximately ten thousand ohms. This is within the expected resistance range for the entire chain. As a result, measuring the resistance of the entire via chain may fail to identify whether the chain has vias with much larger than expected resistances.
Another problem with this approach is that the manufacturers often cannot detect smaller variations in the resistances of the vias. For example, measuring the resistance of the entire via chain may fail to identify whether individual vias have a resistance of two or three times the expected resistance. These vias within operational circuits may still be an operating or reliability issue, but measuring the resistance of the entire chain may fail to identify those vias.